Qui-Gon Jin’s Ghost
A final gift to the fans would be the Force ghost of Qui-Gon Jin. He appears to Obi-Wan in Revenge of the Sith, but a proper conversation between the two would bring closure to his story. He could give Obi-Wan advice to help guide him when he reveals the destiny Luke must face.
The ghost of Qui-Gon Jin would also be a good way to explain why Yoda and Obi-Wan didn’t try to train Luke’s twin sister, Leia. Yoda only mentioned it because Obi-Wan thought Luke would fail. Having the two explain this would help, especially with the “too old to begin the training” thing.
Assuming Disney moves forward, expect to see it sometime in 2020.